Manufacture of variable stiffness microtubing

ABSTRACT

A method for manufacturing novel composite microtubing with variable stiffness over the length of the tubing and a continuous manufacturing process.

This application is a division of copending application Ser. No.08/480,411 filed Jun. 7, 1995 which is a continuation-in-part ofcopending application Ser. No. 08/331,280 filed Oct. 28, 1994.

The present invention relates to the manufacture of microtubing and moreparticularly to microtubing of variable stiffness over the length of thetubing. The microtubes of the present invention are typicallymanufactured at least partially of cured resin and are of outer diameterless than 0.225 inches (5.715 mm). The microtubes of this invention mayusefully be employed in a variety of applications such as medicalcatheters for various diagnostic and therapeutic purposes.

BACKGROUND OF THE INVENTION

Fine gauge microtubing has been made for many years by coating a surfacetreated copper mandrel wire with one or more suitable curable resins andsubsequently removing the mandrel wire after the resin coating has beencured. In this regard, attention is directed to U.S. Pat. No. 4,051,284issued to Ohkubo, et al. on Sep. 27, 1977, and entitled "Method forProducing Heat Resistant Synthetic Resin Tubes," the entire content ofthis prior U.S. Patent being expressly incorporated by reference.

In medical applications, such as guide catheters, it is usuallydesirable that the hollow tube or microtube portion of the catheter havecharacteristics which vary over the length of the tube. Characteristicsthat are particularly desirable along various portions of catheter tubesinclude torque transmission or pushability, stiffness or flexibility,burst strength, and kink resistance. It is also necessary that thecomponents of microtubes used in catheters be bio-compatible so as notto induce thrombosis or other trauma when used.

SUMMARY OF THE INVENTION

The present invention comprises methods of manufacturing novelmulti-layer resin cured microtube and even single layer resin curedmicrotubes which vary in flexibility over their length. Resin curedlayers of the microtube are generally comprised of polyimides,fluoropolymers, polyethylenes, NYLON, urethanes and polyurethanes, andsuch layers may be interspersed with one or more layers of coiled orbraided metal wire or ribbon, or fibers, such as particularly glass,plastic or aramid fibers. The novel microtubes are manufacturedaccording to a continuous process and selected layers or portions oflayers may be removed by grinding or etching portions of the tubes. Inaddition, the number of braid picks per inch and the diameter of thetubing may vary along portions of the tube. By varying the materialscomprising the layers of the microtubes, and in some instances thethickness of those layers, together with the braid pick count andmicrotubing diameter and shape, as well as selectively removing portionsof resin or braid layers, it is possible to achieve variations in tubingstiffness on the order of over 100 to 1. In other words, for a givenlength of microtube, the proximal end may be over 100 times stiffer thanthe distal end. To obtain such wide variations in stiffness previouslyit has been necessary to fabricate tubing from separate tubingcomponents. Such fabrication is not only time consuming and expensive,but the joints between tubing components are especially likely to failor be prone to kinking.

In preferred embodiments of the present invention, the compositemicrotubes have wall thicknesses of about 0.0025 inches (0.0635 mm) toabout 0.01 inches (0.254 mm), inner diameters of 0.005 inches (0.127 mm)or even less to about 0.2 inches (5.08 mm), and outer diameters of 0.01inches (0.254 mm) to 0.22 inches (5.588 mm). Braid pick counts per inchmay range from as few as 30 to 45 picks/inch up to as many as 280picks/inch. The inner diameter of the microtubing may also vary so thatthe widest diameter is as much as twice the size as the narrowestdiameter. When the layers, braid pick count, or diameter of thecomposite microtubing are varied, such variations preferably take placegradually over a length of approximately 1 inch or more to reduce thelikelihood of kinking in the microtubing.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1A is a flow diagram representing the steps employed inmanufacturing microtubing in accordance with the teachings of thepresent invention.

FIG. 1B is a side elevation view of a section of standard wire mandrel.

FIG. 1C is a side elevation view of a section of tapered wire mandrel.

FIG. 2 is an enlarged partial side sectional view of a length ofmicrotubing made according to the present invention in which thematerials comprising the layers of the microtubing walls are varied overdistance.

FIG. 3 is an enlarged partial side sectional view of a length ofmicrotubing made according to the present invention with variable pickcount braiding.

FIG. 4 is an enlarged partial side sectional view of a length ofmicrotubing according to the present invention which is tapered and inwhich the braid layer is selectively removed.

FIG. 5 is an enlarged partial side sectional view of a length of taperedmicrotubing according to the present invention in which both thematerials and the pick count of braiding are varied over distance.

FIG. 6 is an illustration of the braiding area of a braiding machineshowing the guides which can be used when the pick count is varied.

FIG. 7 is an enlarged partial side section view of a single layer resincured microtube according to the present invention.

FIG. 8 is an enlarged partial side sectional view of a compositemicrotube according to the present invention in which the materialencapsulating the braid is changed over the length of the tube.

FIG. 9 is an enlarged partial side sectional view of a tapered compositemicrotube according to the present invention with an extruded outerlayer.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, flow diagram 10 shows in graphic form the various stepsemployed in manufacturing the novel composite microtubing of the presentinvention. A wire mandrel 29 is supplied from wire mandrel supply 11 toan oxidation station 12 which will typically oxidize the outer surfaceof the wire mandrel 29 by a heat or chemically induced reaction. Theoxidation allows mandrel 29 to be easily removed after layers of resinand/or braid materials have been formed on the mandrel 29. In lieu of anoxidized mandrel, which typically is a copper wire, it is also possibleto use some wire mandrels in an unoxidized state such as silver platedcopper wire or annealed stainless steel wire. The oxidized wire mandrel29 then proceeds through a first resin bath 13 preferably of heatresistant synthetic varnish such as polyamide, polyamideimide,polyesterimide, polyesteramideimide, or a fluoropolymer. Preferredcommercially available resins include the PYRE ML-5019 series ofpolyamide varnishes, the polyamic acid solutions of P.D. George ChemicalCompany of St. Louis, polytetrafluoroethylene (PTFE) and fluorinatedethylene propylene (FEP). PTFE or FEP fluoropolymers are typically usedto provide a lower friction interior or exterior surface for theresulting microtubing. Although usually used for outer layers,polyurethane varnishes may also be used. The preferred polyurethanes areTECOFLEX solvent grade polyurethanes from Thermetics.

After the resin coated wire mandrel 29 is heat cured in curing oven 14,it may optionally be spooled or stored before further processing. Theresin coated wire mandrel 29 may also optionally proceed across anetching solution 80 such as illustrated in tank 15. Controller 16 willselectively raise and lower dipping roller 17 to submerge a portion ofthe resin coated wire mandrel 29 in the etching solution 80. For apolyamide resin, the etching solution 80 would typically comprise aheated strong base solution with pH of about 14, such as a sodiumhydroxide solution, a strong heated acid with pH of about 1, or 956 MLStripper available from Fidelity Chemical Company, or equivalents. Aless desirable alternative to etching is to mechanically remove aportion of the unused resin layer, as by centerless grinding. Centerlessgrinding is usually only acceptable for tubing with relatively thickwalls such as would be formed by an extrusion process. If a TEFLON resinis used, it may be treated with Poly Etch™, available from Matheson Gas,which does not remove the cured TEFLON but does improve the adhesion ofsubsequent layers of polyamide resin. The FEP and PTFE resins, as wellas polyethelyne resins are generally unsuitable for selective removal byetching.

It should be made clear that multiple layers of the same or differentresins may be placed on the wire mandrel 29 by drawing the resin coatedmandrel 29 through additional resin baths, curing ovens, and optionallyacross etching tanks. The next process step is to pass the resin coatedwire mandrel 29 through a braider 18 such as a STEEGER braider machine,available from Wilhelm Steeger GmbH. To provide maximum variability itis desirable that the braider 18 be programmable to vary the capstan andcarrier speeds and guide locations in accordance with a predeterminedpattern, and as explained in greater detail in connection with FIG. 6.

The braider 18 typically utilizes between 8 and 32 strands of metalribbon having a thickness between 0.0003 inches (0.000762 mm) and 0.003inches (0.00762 mm) and a width between 0.0025 inches (0.00635 mm) and0.01 inches (0.0254 mm), or alternatively round, D shaped, or other wirewith diameter between 0.001 inches (0.00254 mm) and 0.004 inches(0.01016 mm). In other instances, fibers, most typically glass, plasticor aramid fibers such as KEVLAR may be used as a braiding material. Thenumber of strands braided around the resin coated mandrel 29 istypically 16, but that number would often be reduced if the diameter ofthe mandrel 29 was 0.02 inches (0.0508 mm) or less. Similarly, more than16 strands might be used if the mandrel were of diameter 0.1 inches(0.254 mm) or more. Typically, braiding will involve one strand of braidwire to a carrier on the braiding machine but it is possible to thread asingle carrier with two or more wires for different braidingcharacteristics.

It should be understood at this point that in some cases in lieu ofbraiding, the resin coated mandrel 29 may simply be coiled with wire,ribbon, or fiber to achieve similar results. Braiding is generallypreferred, however. Braiding or coiling the mandrel is not required forall types of microtubing. In some instances microtubing with varieddiameter and materials may provide a sufficient change in stiffness tobe effective.

After braiding, the braided resin coated mandrel 29 is optionally passedover tank 19 with a suitable etching solution 81 (or alternatively amechanical braid removal station). For steel wire or ribbon braid thepreferred solution is a salt water solution through which electriccurrent is passed to achieve electrochemical machining of the immersedbraid. Controller 20 will selectively raise and lower dipping roller 21to submerge selected portions of the resin coated mandrel 29 in thesaline etching solution 81. The result is the selective removal ofportions of the braid layer.

To mechanically remove a braid section, it is desirable that the braidto be removed consist of straight wires. Accordingly, the braid sectionon a length of microtubing would be likely to have a low pick count atthe stiff proximal end, a high pick count at a more flexible distalportion, and straight wires (achieved by stopping the rotation of thecarriers while continuing to pull wires with the capstan). A thin resinlayer referred to as the braid matrix layer is then coated over thebraid and selectively removed by etching the areas where the straightwires are located. The straight wires can then be cut with wire cuttersand fatigued to failure at the interface with the braid matrix layer.This process leaves relatively few irregularities such as wire burrs.

The braided resin coated mandrel 29 is then passed through another resinbath 22, curing oven 23 and optional tank 24 containing an etchingsolution and with controller 25 and dipping rod 26 for submergingselected portions of the composite coated mandrel 29. The resin layerencapsulating the braid is referred to as the braid matrix layer. Byselected removal of portions of the initial matrix layer, and passingthe resin coated mandrel 29 through another resin bath and curing oven,the braid can be encapsulated with different resins over predeterminedlengths of microtubing. By using a relatively stiff material for thebraid matrix at one end of a length of microtubing such as polyamide orPBAX grade NYLON, and a relatively flexible material such as FEP, PTFEor a low durometer polyurethane at the opposite end, the flexibilitychange over the length of the microtubing can be further enhanced.

One layer of resin cured material encasing the braid may be sufficientlythick to provide structural integrity to the resulting compositemicrotube, while still conforming generally to the texture of the outersurface of the woven braid. The exterior surface texture or roughness ofsuch a composite microtube exhibits less drag when used as a guidecatheter than a comparable smooth surfaced material. It will beunderstood that the mandrel 29 may also be coated with additional layersof resin at this stage before proceeding to the final step 27.Additional layers may create a smooth surfaced composite or provideadditional thickness which can be ground smooth. In some instances, itis desirable to extrude the outer layer of the tubing. This isaccomplished by passing the microtubing through an extruder, such as isused to manufacture thicker walled catheter tubing. It is also possibleto vary the flexibility of the extruded layer by using a co-extruderthat can switch between different materials over predetermined lengthsof extrusion. With such a co-extrusion process a relatively highdurometer material may be used for some segments and then transitionedto a relatively lower durometer to provide greater flexibility. Extrudedlayers would typically be used to provide additional wall thickness forthe resulting microtubing, which has the effect of reducing the tendencyof the tubing to kink. Extrusion may also be a more desirable process touse with some particular materials that are desirable for propertiessuch as high flexibility, non-reactivity with respect to tissue, andnon-thrombogenicity with respect to blood.

In the final step 27 the mandrel 29 and the composite tubing that coatsthe mandrel 29 are cut into desired lengths, the mandrel 29 is removedfor recycling thereby defining a central lumen, and the resultingcomposite microtubing is cleaned and finished.

The traditional mandrel 29 is a wire of uniform diameter as illustratedin isolation in FIG. 1B. However, FIG. 1C illustrates a new mandrel 30consisting of a continuous length of wire which has been manufactured orcenterlessly ground to produce tapered segments a from wide point 31 tonarrow point 32. Composite microtubes can be manufactured on thesemandrels 30, the short segments b between narrow points 32 and widepoints 31 being cut out and discarded, while the long segments a, havethe mandrel removed for recycling, and are thereafter finished in thetraditional manner. In this fashion, novel composite tapered microtubescan be manufactured in a continuous process.

In lieu of using tapered mandrels, it is also possible in some instancesto produce tapered microtubes by heating and stretching selectedportions of the microtubing. Typically this is accomplished by cuttingthe microtubing into segments, hanging the segments with a weight at oneend, and heating portions of the microtubing segments, preferably with atubular heater.

It will be understood that other mandrel shapes may also be desired. Forinstance, an elliptical or oval mandrel or a tapered oval mandrel mightbe used.

FIG. 2 illustrates an enlarged partial sectional view of a compositemicrotubing 35 according to the present invention. The proximate end 36of the microtube 35 shows the exterior cured resin surface 42 of themicrotube 35 conforming generally to the texture of the underlying wirebraid, as shown by bumps 37. Wire 39 is in a cloth weave pattern andencased in the outer resin cured layer 38. Inner cured resin layer 40 isbeneath the wire braid but has been removed at the distal end 41 ofmicrotube 35. Accordingly, because the inner cured resin layer 40 ispresent in the proximate end 36 and removed at the distal end 41, themicrotube 35 exhibits greater stiffness at its proximate end than at itsdistal end. Preferably inner cured resin layer 40 does not stopabruptly, but tapers off over about 1/4 inch to 2 inches, and preferablyabout 1 inch, so that the flexibility of microtube 35 changes somewhatgradually and does not exhibit a heightened tendency to kink at the endof the inner cured resin layer 40. Cured resin layers 38, 40 and braid38 define the composite wall 43, the inner surface of which defineslumen 34 extending axially through microtube 35.

FIG. 3 illustrates an alternative microtube 45 according to the presentinvention. The proximate wall 46 of microtube 45 again shows an exteriorcured resin surface 53 conforming generally to the texture of theunderlying wire braid. The interior surface of wall 46 defines lumen 44extending axially through microtube 45. Wires 48 are braided in aclothing weave varying from a relatively low pick count at position 51to a relatively higher pick count at position 52. Wires 48 are embeddedin outer cured resin layer 47. The wire braid is exterior of inner curedlayer 49, which in this illustration proceeds the entire length of themicrotube to distal end 50 of microtube 45. It has been discovered thatincreasing the pick count of the wire braid also increases theflexibility of the microtube. Accordingly, microtube 45 is relativelystiffer at proximate end 46 and position 51 where the pick count isrelatively low, as contrasted with position 52 and the distal end 50where the pick count is relatively high. An additional novel feature isthat the pick count can be varied substantially, for instance from lessthan 50 picks per inch to 280 picks per inch when using 0.0015 inchdiameter round steel wire braid, along a one inch segment of microtubingby using the wire guides described in FIG. 6. If the braid layer isreplaced with coiled wire, ribbon, or fiber, similar variations instiffness are achieved by altering the pitch of the coiling materialwith respect to the resin coated mandrel as it is wound. When the pitchapproaches 90°, the greatest flexibility is achieved; when the pitchapproaches 0°, the greatest stiffness is realized.

FIG. 4 shows a tapered microtube 55 with the diameter of lumen 54encompassed by proximate wall 56 being greater than the diameterencompassed distal wall 60. Also shown are inner cured resin layer 59,and wire 58 braided in a clothing weave and encased in outer cured resinlayer 57. It has been established that stiffness varies proportionatelyto the third power of the diameter of a thin walled microtube andproportionately to the fourth power of the diameter of a thick walledtube. Accordingly, if the diameter of microtube 55 at proximate end 56is twice the diameter at distal end 60, the proximate end 56 ofmicrotube 55 will be approximately 8 times as stiff as distal end 60. Itwill be recognized that it is possible to combine a reduction in amicrotube's diameter together with increasing the pick count of a wirebraid layer, and etching away part or all of at least one layer of resinor braid from a portion of the microtube. In this fashion, a microtubeconstruction is possible in which the proximate end is more than 100times as stiff as the distal end, and the microtubing still providessatisfactory strength and resistance to kinking throughout.

Furthermore exotic variations are possible with other microtubingshapes. For instance, an elliptical or oval shaped microtube has greaterflexibility in the direction of its minor axis and greater stiffness inthe direction of its major axis. A tapered oval shape combines thecharacteristics of oval microtubing with the reduction in stiffnessachieved by reducing the overall diameter of the microtubing.

FIG. 5 illustrates a microtubing 62 with a combination of the previouslydescribed techniques. Microtubing 62 has a proximate end 63 withinterior diameter d and a smooth outer resin wall 64 encasing the wovenwires 65, 68 which comprise a braid layer. The braided wires 65, 68 arewoven over an inner resin layer 66 that commences at the proximate end63 of the microtube 62 but which is etched or ground away as itapproaches the distal end 67 of the microtube. In the illustratedembodiment, the etching has completely removed the inner resin layer 66at the distal end 67. The pick count of the woven wires 65, 68 alsovaries, being relatively lower at the proximate end 63 and at wires 65than at the distal end 67 and wires 68. The removal of inner resin layer66 and the increased pick count toward the distal end 67 make the distalend 67 of the microtubing 62 more flexible than the proximate end 63.Adding further to the flexibility of the distal end 63 is that thedistal inner diameter d' is approximately one half the proximal diameterd.

The changes in stiffness that can be realized with novel microtubingsaccording to the present invention are exemplified by the samples testedin the following table:

                  TABLE I    ______________________________________    Distance    From   Inner    Outer           Stiffness                                           Kink    Proximal           Diameter Diameter        *      Diameter    End (cm)           (mm)     (mm)     Picks/cm                                    gm/cm  (cm)    ______________________________________    Tube A - Inner layer polyamide, Stainless Steel braid .0015    inch diameter, Outer layer PTFE    5.1    .757     .965     14.2   117.7  3.17    20.3   .757     .965     17.3   118.0  3.17    35.6   .757     .968     18.1   82.3   2.54    50.8   .759     .975     22.0   56.2   1.27    66.0   .759     .978     22.8   42.8   1.02    81.3   .762     .978     26.8   37.7   1.02    91.4   .765     .996     52.8   21.2   .51    111.8  .765     .996     56.7   23.1   .51    127    .765     .993     53.5   26.0   .51    Tube B - Inner layer polyamide, Stainless Steel braid .0015    inch diameter, Outer layer PTFE    5.1    .879     1.069    13.8   129.3  3.81    21.6   .881     1.077    15.0   112.9  2.54    38.1   .889     1.080    19.7   63.4   1.02    54.6   .889     1.085    22.8   56.9   1.02    71.1   .894     1.097    25.1   43.7   .76    87.6   .897     1.105    45.7   37.8   .51    102.9  .897     1.105    56.7   40.3   .51    Tube C - Inner layer PTFE, Second layer-polyamide (1),    Stainless Steel braid .0015 inch diameter, Outer layer FEP    5.1    .762     1.016    19.7   52.9   1.52    50.8   .762     1.016    110.2  9.1    .51    Tube D - Inner layer polyamide, Stainless Steel Braid .0015    inch diameter, Outer Layer FEP    5.1    .867     1.041    19.7   41.26  1.52    50.8   .635     .867     110.2  17.68  .51    Tube E - Inner Layer PTFE, Second Layer polyamide (1), stainless    steel braid 0.0007 inches × 0.003 inches Outer Layer Tecoflex 93A    5.1    0.711    0.94     15.7   128.8  1.27    114.3  0.711    0.889    52     4.3    <0.3175    ______________________________________     * Measured on a 1.27 cm (0.5 inch) segment centered at the specified     distance from the proximal end.     (1) Second layer is removed by etching at about 40 cm from the proximal     end

FIG. 6 shows the braiding area 70 of a typical braiding machine, withresin coated mandrel 73 proceeding upward through pick down guide 71,and then being braided with wires 74 and proceeding through pick upguide 72. The braided resin coated mandrel 75 then proceeds to a take upreel or capstan. The braider is preferably controlled by a programmablelogic chip so that the capstan speed, which controls the speed of themandrel 73 through the braiding area, and the carrier speed, whichcontrols the speed with which wire is braided or wrapped around themandrel 73, can be altered when desired. Typically, a mandrel 73 will beeither braided at a uniform rate, providing a braid layer with uniformpick count, or else at a variable rate repeating over a specifieddistance. The variable rate braided layer will have varied pick countsrepeating over the specified distance. When the mandrel 73 has beencompletely processed so that all layers of braid and resin are finished,the mandrel 73 or tubing will be cut into lengths matching the specifieddistance and finished to provide microtubes with pick counts varyinguniformly from the proximal end of each finished microtube. Theresulting variable stiffness composite microtubes will each have thedesired predetermined stiffness pattern varying over the length of thosemicrotubes.

In the braiding areas, the wires being braided are at an angle α to theresin coated mandrel as they are wrapped. When the pick count increases,angle α increases. When the pick count decreases, angle α decreases.However, if the pick count of the braid is suddenly increased, up fromsay 45 to 90 picks per inch, the mere slowing of the capstan speed andincrease of the carrier speed is not sufficient to immediately changethe pick count. Instead, as angle α increases, the zone 76 in which thewires 74 contact the resin cured mandrel 73 moves higher and the fullincrease in pick count is only achieved gradually over about a one footlength. It is desired that more rapid pick count changes be realized forsome tubing construction. This result is achieved by the use of apick-up guide 72 and a pick-down guide 71.

To keep the zone 76 at which wires 74 contact resin cured mandrel 73relatively constant and thereby more rapidly achieve the full increasein pick count, pick-up guide 72 can be lowered to keep the wires 74contacting mandrel 73 in zone 76. A similar problem occurs when the pickcount is lowered. The pick-down guide 71 can be raised to keep the wires74 contacting mandrel 73 in the same zone 76. Use of pick-up andpick-down guides allows significant pick count variations to be achievedover relatively short distances such as one inch (2.54 mm). It is notgenerally desirable to vary the pick count greatly over distances muchshorter than one inch because of the increased tendency of the resultingmicrotubing to kink.

The pick-up guide 72 and pick-down guide 71 achieve their desiredresults by changing the radius of the guide for the Steeger Braider fromapproximately 12 inches to the much smaller dimension of approximately0.25 inches. Braider kinematics equations set forth in detail in"Processing Model of Circular Braiding," Processing of Polymers andPolymeric Composites, MD-V19, ASME 1990 by Guang Wu-Du, Peter Popper andTsu-Wei Chen, show that convergence length (the distance over which apick count change is completed) due to a carrier or capstan speed changeis minimized by making the guide radius as small as possible. Since bythe use of the pick-up guide 72 and pick-down guide 71 rapid pick countchanges can be achieved, near linear pick-count changes can also beachieved by changing either the capstan or carrier speeds in a linearfashion utilizing a programmable electronic controller.

FIG. 7 illustrates a microtubing 102 with only a single layer of curedresin 104. Microtubing 102 has a proximate end 103 with interiordiameter d. Microtubing 102 narrows toward its distal end 107 to innerdiameter d'. The cured resin layer 104 has also been etched toward thedistal end 107 so that the thickness of the cured resin layer 104 isdiminished.

FIG. 8 illustrates a multilayer microtubing 82 of uniform diameter.Microtubing 82 has a proximate end 83 and a distal end 87. Microtubing82 is comprised of an inner resin cured layer 86 that commences at theproximate end 83 and extends to the distal end 87. Woven wires 85, 88comprise a braid layer over the inner resin cured layer 86. The pickcount of the woven wires 85, 88 also varies, being relatively lower atthe proximate end 83 and at wires 88 than at the distal end 87 and wires85. The intermediate resin cured layer 89 commences at the proximate end83 of the microtube 82 and comprises the braid matrix layer at theproximate end 83, but is etched away as it approaches the distal end 87.An outer layer 84 proceeds the entire length of microtube 82 andcomprises the braid matrix layer at the distal end 87 where theintermediate resin cured layer 89 was removed.

FIG. 9 illustrates a microtubing 92 with a combination of the previouslydescribed techniques and with outer extruded layer 91. Specifically,microtubing 92 has a proximate end 93 with interior diameter d and asmooth intermediate resin wall 94 encasing the woven wires 95, 98 whichcomprise a braid layer. The braided wires 95, 98 are woven over an innerresin layer 96 that commences at the proximate end 93 of the microtube92 but which is etched or ground away as it approaches the distal end 97of the microtube. In the illustrated embodiment, the etching hascompletely removed the inner resin layer 96 and the intermediate resinlayer 99 toward the distal end. In addition, the pick count of the wovenwires 95, 98 also varies being relatively lower at the proximate end 93and wires 95, and increasing toward wires 98. The woven wires 95, 98 arecompletely removed either manually or preferably by electrochemicalmachining toward the distal end 97. The increased pick count towards thedistal end 97 and complete removal of the wires 95, 98 at the distal end97 makes microtubing 92 progressively more flexible toward the distalend 97. In addition, outer extruded layer 91 may be a soft and flexiblematerial such as polyethylene or polyurethane. The removal of innerlayer 96 and intermediate layer 99 toward the distal end 97 allowsadditional flexibility at the distal end 97 as does the reduced distaldiameter d' relative to the proximal diameter d.

While the invention has been described in terms of its preferredembodiments, modifications obvious to one having ordinary skill in theart may be made without departing from the scope of the invention whichare intended to be covered by the appended claims.

We claim:
 1. A process for manufacturing predetermined lengths ofmicrotubing comprising the steps of(a) coating a continuous wire mandrelwith a first curable resin layer; (b) curing said first curable resinlayer; (c) optionally removing segments of said first cured resin layerat each predetermined length; (d) coating said continuous mandrel with asecond curable resin layer; (e) curing said second curable resin layer;(f) removing segments of said second cured resin layer at eachpredetermined length; (g) cutting said resin coated mandrel at eachpredetermined length and removing said mandrel so that the remainingcured resin defines a microtube.
 2. The process of claim 1 wherein saidcontinuous wire mandrel is tapered over a portion of each predeterminedlength.
 3. The process of claim 1 wherein the continuous wire mandrelhas an elliptical cross section.
 4. The process of claim 1 furthercomprising the steps of braiding a braid layer over said first curedresin layer, and optionally removing segments of said braid layer ateach predetermined length.
 5. The process of claim 4 wherein thebraiding of the braid layer varies from a relatively low pick count to arelatively high pick count in accordance with a predetermined patternreporting each predetermined length.
 6. A process for manufacturingpredetermined lengths of microtubing comprising the steps of(a) coatinga continuous wire mandrel with a first curable resin layer; (b) curingsaid first curable resin layer; (c) optionally removing segments of saidfirst cured resin layer at each predetermined length; (d) braiding abraid layer over said first cured resin layer; (e) removing segments ofsaid braid layer at each predetermined length; (f) coating saidcontinuous mandrel with a second curable resin layer; (g) curing saidsecond curable resin layer; (h) cutting said resin coated mandrel ateach predetermined length and removing said mandrel so that theremaining cured resin and braid defines a microtube.
 7. The process ofclaim 6 wherein the braiding of the braid layer varies from a relativelylow pick count to a relatively high pick count in accordance with apredetermined pattern repeating each predetermined length.
 8. A processfor manufacturing predetermined lengths of microtubing comprising thesteps of:(a) coating a continuous wire mandrel with a first curableresin layer; (b) curing said first curable resin layer; (c) optionallyremoving segments of said first cured resin layer at each predeterminedlength; (d) braiding a braid layer over said first cured resin layer;(e) coating the continuous wire mandrel with a second curable resinlayer; (f) removing segments of said second cured resin layer at eachpredetermined length; (g) removing segments of the braid layer exposedby the removal of segments of the second cured resin layer in step (f);(h) cutting said mandrel at each predetermined length and removing saidmandrel so that the remaining cured resin defines a microtube.
 9. Theprocess of claim 8 wherein the braid layer is comprised of a pluralityof metal wires and wherein the braiding of the braid layer includes asection where the wires are pulled parallel to the continuous wiremandrel beneath at least some of the segments of the second cured resinlayer that are removed in step (f).
 10. The process of claim 9 whereinmetal wires exposed by the removal of segments of the second cured resinlayer in step (f) are physically removed by a method selected from thegroup consisting of cutting the wires with wire cutters and of fatiguingthe wires to failure.
 11. A process for manufacturing predeterminedlengths of microtubing comprising the steps of:(a) providing acontinuous wire mandrel tapered over a portion of each predeterminedlength; (b) coating said continuous wire mandrel with a first curableresin layer; (c) curing said first curable resin layer; (d) braiding abraid layer over said first cured resin layer; (e) coating thecontinuous wire mandrel with a second curable resin layers: (f) cuttingsaid mandrel at each predetermined length and removing said mandrel sothat the cured resin and braid defines a microtube.
 12. The method ofclaim 11 wherein a segment of at least one of said first curable resinlayer, said braid layer, and said second curable resin layer is removed.13. A process for manufacturing predetermined lengths of microtubingcomprising the steps of:(a) coating a continuous wire mandrel with afirst curable resin layer; (b) curing said first curable resin layer;(c) braiding a braid layer over said first cured resin layer such thatthe Pick count varies from a relatively low Dick count to a relativelyhigh Dick count in accordance with a predetermined pattern over eachpredetermined length; (d) coating the continuous wire mandrel with asecond curable resin layer; (e) curing said second curable resin layer;(f) cutting said mandrel at each predetermined length and removing saidmandrel so that the cured resin and braid define a microtube.
 14. Themethod of claim 13 wherein a segment of at least one of said firstcurable resin layer, said braid layer and said second curable resinlayer is removed.
 15. A process for manufacturing predetermined lengthsof microtubing comprising the steps of:(a) providing a continuous wiremandrel of ellipsoid cross-section; (b) coating said continuous wiremandrel with a first curable resin layer; (c) curing said first curableresin layer; (d) braiding a braid layer over said first cured resinlayer; (e) coating the continuous wire mandrel with a second curableresin layer; (f) cutting said mandrel at each predetermined length andremoving said mandrel so that the cured resin and braid defines amicrotube.
 16. The method of claim 15 wherein a segment of at least oneof said first curable resin layer, said braid layer and said secondcurable resin layer is removed.